Plant the Stonemaul Banner near the entrance to Onyxia's Lair and defeat anyone who answers.
Brogg relishes the thought of taking his revenge on the black dragonkin of the marsh. <name>, you have been very helpful in preparing for this moment. Will you help?
Take Brogg's clan banner and travel east until you see a cave like a dragon's mouth. At the entrance to that cave, plant Stonemaul banner and issue Brogg's challenge to Onyxia.
Brogg and the Stonemaul will have their lands back!